Variable-capacity element



June 4,

c. T. ALLCUTT VARIABLE CAPACITY ELEMENT INVENTCR (he fer 7.'/4//c1/flFiled Sept. 1, 1921 MMS 8 WITNESSES:

ATTORNEY Patented June 4 1929.

UNITED STATES PATENT OFFICE.

HOUSE ELECTRIC & MANUFACTURING COMPANY,

VAN IA.

A CORPORATION OF PENNSYL- VARIABLE-CAPACITY ELEMENT.

Application filed September 1, 1921. Serial No. 497,553.

My invention relates to electric devices and more especially tovariable-capacity electric devices.

One of the objects of my invention is to provide an electrical device,the capacity of which is a function of the voltage impressed thereon.

Another object of my invention is to provide a circuit arrangement,whereby my device may be employed as a frequency multip ier.

As is well known, the capacity of condensers, as heretofore employed, isfixed in theconstruction of the condenser. That is to say, the capacityof the condenser is independent of the voltage impressed across itsterminals. It may readily be seen, however, that, by employing acondenser wherein the capacity is a function of the voltage, greaterflexibility may be attained in certain types of static net-works.

According to my invention, I provide a capacitive reactance device inwhich the capacity is a function of the voltage impressed thereon andwhich may be applied in applications similar to inductive reactancedevices utilizing the magnetic-saturation properties of iron cores.

The variable-capacity element embodying my invention comprises anevacuated electric device having a central cathode and a coaxiallymounted anode contained therein. In addition, means are provided forproducing a substantially constant magnetic field transverse to theelectron path between the anode and the cathode, all as will beexplained more fully hereinafter. This magnetic field causes the movingelectrons to travel in a curved path. The field must be made ofsufficient strength to cause the I electrons to curve so much that theywill return to the cathode or to the neighborhood of the cathode withouthaving reached the anode.

As the electrons travel in their curved paths they approach and thenrecede from the anode without actually reaching it. They thus constitutea space charge. This charge is located within the surface thatconstitutes the geometrical envelope of the scveral curved paths. Thewhole space within this surface may be regarded as the cathode since itis the seat of the charge. The effect, therefore, of the presence of thetraveling electrons which do not reach the anode is equivalent to anincrease in the size of the cathode. This effect may be regarded as achange in the eifective diameter of the cathode. Because of the presenceof a considerable number of traveling electrons in the space between thefilament and the said geometrical envelope, it may be regarded as aconductive space, while the space between this geometrical envelope andthe anode, being free or almost free from electrons, is not conductive.

The space charge, acting as an equivalent to an increase in the diameterof the cathode, is similar to a reduction of the distance separating thetwo electrodes. In any condenser the distance between the electrodes isone of the factors determining the capacity. The ca acity increases asthis distance diminishes.

hose and other objects of my invention, as well as details ofconstruction, whereby my invention may be practiced, will appear morefully in the followin description, when read in connection with t eaccompanyingr drawing, wherein:

ig. 1 is a diagrammatic view of circuits and apparatus embodying myinvention, as applied, particularly, to a frequency-doubler system.

Fig. 2 is a curve diagram showing the relation between the voltagesimpressed across the terminals of the variable capacity element, asordinates, and the charges on the electrodes contained therein, asabscissae.

Fig. 3 is a series of curve diagrams explaining the theory of operationof the frequency-doubler system of Fig. 1.

Fig. 4 is a cross-sectional view of the tube shown in Fig. 1,illustrating the direction of the electron path when the transversemagnetic field is substantially zero, and

Fig. 5 is a similar view showing the effect of a relatively largetransverse magnetic field upon the electron path.

The operation of the variable-capacity element embodying my invention isbased upon the effect of a transverse magnetic field upon the path of anelectron emitted from a hot cathode. In a vacuum-tube device 1, such,for example, as that shown in Fig. 1 and comprising a central cathode 2and a coaxially mounted anode in the forinof a split cylinder 3, theelectrons emitted from the cathode travel in substantially straightlines to the anode. Such condition is illustrated in Fig. 4, the dottedlines 4 representing the paths of the electrons.

However, upon the application of a magnetic field transverse to thepaths of the electrons, it is found that the paths become curved orcycloidal in shape and that the degree of curvature depends upon theintensity of the transverse magnetic field. Upon the application of amagnetic field of sufiicient intensity, the electrons may be caused tomove around the hot cathode 2 in curved paths without reaching theanode, such condition being illustrated by dotted lines 5 in Fig. 5.

The expression space current path has been used herein to designate thespace between the cathode and the anode through which ordinarily spacecurrent flows. It should be remembered, however, that in this device thecurrent across this space is very small,'if not zero. The paths of theseveral electrons being curved do not cross this space. The spacecurrent path therefore, is not the actual path of the electrons whenthis device is in operation.

With the transverse magnetic field substantially zero, there exists,between the electrodes of the tube 1, a definite capacity which is fixedby the particular construction of the tube. When the transverse magneticfield, however, is adjusted to some such value that the electrons areprevented from reaching the anode, as illustrated in Fig. 5, it is foundthat the capacity between the electrodes is increased by reason of anincrease in the effective diameter of the cathode. The increase in theeffective diameter of the cathode is due, as explained above, to thespace charge. When the difference of potential between the cathode andthe anode increases, the radial velocity of the electrons due to theelectrostatic field increases, and the geometrical envelope of thecurved paths of the electrons comes nearer to the anode. This increasesthe capacity. The capacity of the device, therefore, increases withincreas ing voltage. Ascan readily be seen, the increase in theeffective diameter of the cathode is caused by the electrons which aremoving around the cathode.

Since the diameter of the curved electron paths 5 and so the diameter ofthe geometrical envelope of these paths is dependent upon the intensityof the transverse magnetic field and upon the voltage impressed acrossthe terminals of the tube, it follows that the capacity between theelectrodes may be readily controlled.

In Fig. 1, I have shown one application of the variable-capacity elementembodying my invention, as applied to a frequencydoubler system,although it is not to be limited to such system. A primary circuit 6 ofthe frequency-doubler system comprises netic field is produced by meansof a magnetizing winding 14 which is wound around the tube and which maybe energized from a source of direct-current energy 15 through aresistor 16 and a radio-frequency choke coil 17.

An outgoing circuit 19 comprises a secondary winding 21 of the couplingtransformer 8 and a condenser 22 which is connected in shunt relation tosaid secondary winding,the circuit formed by said shunt connection beingtuned to the double-frequency harmonic which is introduced into theprimary circuit 6 by the variablecapacity element 1, as will beexplained hereinafter.

In Fig. 2, is shown a curve indicating the relation between the voltageimpressed across the terminals of the Variable-capacity element, asordinates, and the charge on the electrodes contained therein, asabscissae. It will be noted that the general shape of the curve issimilar to the well known saturation curve of an iron core. It isimportant to notice that this curve does not show the relation betweenthe voltage and the current. Such relation is shown by the usualcharacteristic curve for a vacuum tube. In this tube the magnetic fieldis strong enough to prevent electrons from reaching the anode and,therefore, the current is always very small, the device acting in thisrespect like a true condenser except that it may have a somewhat greaterleakage.

Since this curve has a very marked knee, it is evident that at acharacteristic potential the rate, of change of the charge upon theelectrodes with changing potential will alter abruptly. In order toavoid lengthy expressions in the specification and claims, this value ofthe potential is called the critical potential. The value of the biasingvoltage of the direct-current source 12 is such as to adjust theconstants of the tube to a point Where operation may be effected uponthe knee of the curve shown in Fig. 2.

The operation of my device, as a frequency-doubler, may be more readilyexplained by referring to the curves shown in Fig. 3, wherein curve ashows the sinusoidal Wave form of the primary voltage impressed acrossthe terminals of the variablecapacity element 1, and curve 6 representsthe variations in the charge on the electrodes corresponding to thevariations in voltage, as shown in curve a.

The reason for the variations in curve I) may be explained by referrinto Fig. 2. Since the biasing voltage is such value as to adjust thecharge upon the electrodes to a value corresponding to the knee of thecurve, the effect of positive half cycles of the voltage wave upon thecharge is many times greater than for negative half cycles, therebyinducing a double-frequency harmonic voltage in the secondary circuit,as shown in the current curve 0. The latter curve is obtained from curveI) by taking the derivative thereof with respect to time.

In view of the foregoing description, it can readily be seen that, uponadjusting the biasing voltage to the knee of the curve shown in Fig. 2,double-frequency currents are introduced in the primary circuit 6 whichmay be absorbed by the double-frequency circuit 19.

An advantage of my invention is the provision of a simple, efiicient andeasily constructed variable-capacity element, the operatingcharacteristics of which are such that it may be employed either as afrequency multiplier or as an amplifier.

While I have shown the variable-capacity element embodying my inventionconstructed according to a special design and em ployed in a specificcircuit arrangement, it is not to be limited thereby and I desire thatonly such limitations shall be imposed there on as are indicated in theprior art or are specifically set forth in the appended claims.

I claim as my invention:

l. A variable-capacity element comprising an evacuated electric devicehaving a spacecurrent path therein, electrodes individual to saidspace-current path and means associated with said device for producing amagnetic field transverse to said space-current path and of sufficientstrength to prevent the electrons from completely traversing the spacebetween said electrodes.

2. A variable-capacity element comprising an enclosed envelope, aspace-current path, electrodes individual to said space-current path andmeans for producing a substantially constant magnetic field transverseto said space-current path and of suflicient strength to prevent theelectrons from completely traversing the space between said electrodes.

3. A variable-capacity element wherein are combined a hot cathode, ananode and means for producing a magnetic field substantially constant invalue, said magnetic field being transverse to said space-current pathand of sufiicient strength to prevent the electrons from completelytraversing the space between said electrodes.

4. In an electrical system, a variable-capacity element wherein arecombined a containing envelope, a hot cathode, an anode disposed insymmetrical relation thereto and magnetizing windings symmetricallydis-- posed with respect to said anode and included in a circuitcontaining a direct-current source of energy, said direct current beingsufficient to produce a magnetic field of the strength that will preventthe electrons from completely traversing the space between saidelectrodes.

5. In an electrical system, a variable-capacity element consisting of ahot cathode, an anode, means for producing an electron stream from saidhot cathode toward said anode and means for producing a magnetic fieldtransverse to said electron stream, whereby said electrons are caused toassume curved paths, the capacity of said element being a function ofthe diameter of said curved paths.

6. A condenser drawing leading wattless currents having anon-rectilinear voltagecurrent characteristic comprising a pair ofconductors, means .for establishing, between said conductors, aspace-current discharge having a critical-potential condition, and meansfor applying a biasing component unidirectional potential approximatingsaid critical potential.

7. An electrical condenser comprising a pair of separated conductingmedia, at least one of said media having a relatively small diameter ofcross-section as compared to its length, and means for rendering theeffective diameter of said medium variable in accordance with themagnitude of the impressed potential.

8. A device having a variable electrical capacity, which capacity is afunction of the voltage impressed thereon, which comprises an envelopehaving electrodes at the ends of a space-current path, and means forproducing a unidirectional magnetic field trans verse to said path andof sufiicient strength to prevent any electrons from completelytraversing the space between said electrodes.

9. A variable-capacity element comprising a hot cathode, a co-axiallymounted anode, and means for producing a magnetic field substantiallyconstant in value, said magnetic field being transverse to the currentpath between said cathode and anode and of sufficient strength toprevent any electrons from completely traversing the space between saidelectrodes.

10. The method of varying the electrostatic capacity of an evacuatedelectric device having an electron-emissive cathode and anotherelectrode therein which comprises impressing a transverse magnetic fieldbetween said electrodes of such strength that substantially none of saidelectrons reach said other electrode, and varying the potentialimpressed between said electrodes.

11. The method of varying the electrostatic capacity between anelectron-emissive cathode and another electrode which comprisesimpressing upon the space between said electrodes a magnetic field ofsuch intensity as to cause electrons emanating from said cathode totraverse curved paths which do not intersect the other electrode, and

varying the voltage impressed between said electrodes.

In testimony whereof, I have hereunto 10 subscribed my name this 16thday of August, 19.21.

CHESTER T. ALLCUTT.

